According to the present invention there is provided a mass spectrometer and a method of mass spectrometry. Disulfide (S—S) bond mapping is crucial in the development and quality control of biopharmaceuticals since these bonds have a direct effect on protein structure and function. Enzymatic digestion of native proteins followed by LC-MS/MS or LC-MSE analysis can provide information on the location of S—S bonds. However, the analysis is complicated by a combinatoric explosion of possible linkages especially when the protein is cysteine-rich or when other variable post translational modifications are involved. Another problem for data analysis is the large number of charges carried by peptide complexes.
The preferential cleavage of disulfide bonds (c.f. C—S bonds) by a 248 nm laser was noted by Bookwalter in 1995.
It is also known to use a 266 nm YAG laser to preferentially cleave peptide S—S bonds of peptide complex ions held in an ion trap. It is also known to mass analyse the resulting peptide ions which comprise the precursor ions and the individual peptide chains (with a fairly even charge state split).
FIG. 1 illustrates the known process of cleaving a single disulfide bond of a peptide complex to form two peptide sequences. In a similar manner, a peptide complex of three peptide sequences A,B,C linked by two disulfide bonds (i.e. A-B-C) will yield precursor ions (ABC), peptide pairs (AB,BC) and individual peptide chains (A,B,C). By way of contrast, Collision Induced Dissociation (“CID”) fragmentation of a peptide complex produces mass spectra containing many fragments which are much harder to interpret.
It is also known that labile phosphorylation modifications are preserved when peptide complexes are subjected to photo-dissociation i.e. photo-dissociation selectively cleaves the S—S bond without losing the phosphorylation.
It is desired to provide an improved method of mass spectrometry and an improved mass spectrometer.